Tardigrades- The unlikely candidate for life on other planets
Tardigrades are believed to be the most resilient organisms currently known to science, utilising a series of evolved adaptations to survive more extreme conditions than any other organism currently studied. This microscopic phylum consists of over 1000 species, and occupy habitats from 5546m above sea level to over 4000m below. Fossils of tardigrades have been dated back to the Cambrian period, over 500 million years ago; displaying the longevity and hardiness of these organisms. Since their discovery in 1773, tardigrades have intrigued scientists with their abilities to seemingly survive the highest extremes of temperature, salinity, and radiation.
Why are Tardigrades so special?
The most scientifically interesting characteristic tardigrada possess is their ability to survive for prolonged interludes without water. Despite being an organism that consists of 85% water, during these periods their water content drops by 82%; this ‘drying out’ is known as desiccation. They survive through a process called cryptobiosis. By shutting down their metabolism to less than 0.01% of its normal rate they cause all organs and functions to stop. This coma-like state, known as tun, is completely reversible once conditions become favourable for the individual. While it is of great scientific importance, it has only been discovered relatively recently and the limits of this process are yet to be tested. Unsubstantiated claims suggest these organisms can be reanimated after 120 years; modern science has found individuals are able to reanimate and reproduce after 8 years of desiccation, creating new populations.
How is this achieved?
How the tardigrade achieves this tun state is not fully understood; it was once thought that trehalose, a crystalline sugar, was used to protect cell walls and replace the water within the cell, preventing drying out; this has been shown to occur with other phyla that can tolerate desiccation. However, recent research suggests they synthesise unique sugars to serve this purpose, allowing for longer periods of desiccation. The thick chitin plating tardigrades possess is also thought to decrease desiccation. Their DNA is repaired while they are in this tun state by a protein called damage suppressor (DSUP), which ensures the creation of new cells.
Tolerances of Tardigrades
The sugars utilised within cells to prevent dehydration can also limit the effects of salinity stress, as tardigrades are known to occupy environments of high salinities; including within the Southern Ocean, where salinity varies between normal levels at 35PSU up to 209PSU within sea ice. This also causes desiccation.
Cryptobiosis also protects the tardigrade from the extremes of cold. In cold conditions, water contained within cells freezes and expands, causing internal rupture. Tardigrades within lab conditions can survive temperatures lower than -250°F, well below those found in any natural environment on Earth. They may also be able to survive these conditions for lengthy time scales. A study in 1993 found that Tardigrades could be reanimated after spending more than 30 years frozen solid. While only one individual survived, it was immediately able to reproduce.
At 300 megapascals (MPa) of pressure the majority of bacteria and microorganisms die due to damage of cell walls and DNA. Tardigrades in a lab environment survived to 600MPa while in the tun state; these high pressures are 6 times more than any found on Earth, including the deep sea.
Tardigrades have been shown to survive extremes in radiation, reaching levels found in space. In 2007 tardigrades of two species were sent into outer space on the outside of a satellite, exposing them to high levels of radiation, low temperatures and lack of gravity. While not all of the 2000 sent survived, the majority lived and many had reproduced during the process, seemingly suffering no negative effects. Tardigrades have survived radiation levels up to 1000 times more than any human could survive, both in the lab and in the vacuum of space. Any mistakes in DNA replication caused by exposure to radiation are alleged to be corrected by DSUP.
Potential for alien life
The high levels of tolerance tardigrades possess, both for conditions on Earth and Outer space, have lead recent studies to evaluate the potential for alien life to resemble these organisms. New discoveries of ice covered moons, such as Ganymede, are being investigated as potential sites for colonisation of these alien species. Tardigrades have been shown to be pioneer species, colonising areas, providing food sources for other invertebrates and creating a food web where none previously existed. They are common prey for many other animals in these extreme environments. The existence of tardigrade-like organisms may therefore indicate the presence of further species. There is even a theory that all life is descended from similar organisms arriving on Earth within meteorites.
There are issues with this theory, however. Individuals are only found to be considered ‘hardy’ in the tun state, little study has been done on organisms outside of this state. Populations on other planets would need to survive these extremes continually. The experiments referred to throughout this article may not exist in nature, and the singular space experiment has not yet been replicated. This is a relatively new area of study; further investigation will need to be done to understand how and if these species could survive in space. Whenever organisms are found in space, they will not be like anything we imagine as an alien.